Composite material having high thermal conductivity and low thermal expansion coefficient, and heat-dissipating substrate, and their production methods

ABSTRACT

A composite material having a high thermal conductivity and a small thermal expansion coefficient, which is obtained by impregnating a porous graphitized extrudate with a metal; the composite material having such anisotropy that the thermal conductivity and the thermal expansion coefficient are 250 W/mK or more and less than 4×10 −6 /K, respectively, in an extrusion direction; and that the thermal conductivity and the thermal expansion coefficient are 150 W/mK or more and 10×10 −6 /K or less, respectively, in a direction perpendicular to the extrusion direction.

FIELD OF THE INVENTION

[0001] The present invention relates to a composite material having ahigh thermal conductivity and a small thermal expansion coefficient, aheat-dissipating substrate formed therefrom, and methods for producingthem, particularly to a composite material composed of a porousgraphitized extrudate and aluminum or copper and thus having a highthermal conductivity, a small thermal expansion coefficient and smallresistivity with substantially no thermal hysteresis, which is suitablefor heat sinks for electronic equipment, etc., a heat-dissipatingsubstrate such as a heat sink, etc. formed from such composite material,and methods for producing them.

BACKGROUND OF THE INVENTION

[0002] Because electronic parts have been increasing their heatgeneration as their integration, volume, output, etc. are increasing,demand is mounting on materials having a high thermal conductivity and asmall thermal expansion coefficient. Because semiconductor devices suchas CPUs, light-emitting diodes, etc. generate large amounts of heat,heat sinks are usually mounted to them. Heat transmitted from thesemiconductor devices to the heat sinks is dissipated by fans or coolingmedia, etc. The heat sinks are usually made of aluminum, copper or theiralloys having excellent thermal conductivity.

[0003] Because CPU, for instance, is much smaller than the heat sink, ahigh-thermal-conductivity body called “heat spreader” is usuallyinterposed therebetween. Materials for the heat spreader preferably havea high thermal conductivity and as small a thermal expansion coefficientas that of the CPU made of silicon. This is true of light-emittingdiodes made of compound semiconductors (GaAs, GaN, etc.). For suchpurposes, a lot of substrates made of composite materials of ceramicshaving small thermal expansion coefficients such as silicon carbide,alumina, silicon nitride or aluminum nitride and aluminum or copper havebeen proposed. However, substrates made of these composite materials aredisadvantageously poor in workability because of containing ceramics.Though composite material substrates made of metals having small thermalexpansion coefficients such as tungsten or molybdenum and copper havealso been proposed, these composite material substrates aredisadvantageously poor in workability.

[0004] Under the above circumstances, a lot of attempts have recentlybeen proposed to use composite materials of carbon particles or fibersand metals for heat-dissipating substrates. For instance, JP 10-168502 Adiscloses a high-thermal-conductivity composite material obtained bymixing 1 to 200 parts by weight of one or more crystalline carbonmaterials selected from the group consisting of graphite, carbon fibers,carbon black, fullerene and carbon nanotubes, and 100 parts by weight ofmetal powder selected from the group consisting of Fe, Cu, Al, Ag, Be,Mg, W, Ni, Mo, Si, Zn and these alloys, and hot-pressing the resultantmixture. However, because this composite material has a structurecontaining crystalline carbon materials dispersed in a metal matrix, ithas as large a thermal expansion coefficient as that of the metalmatrix, though it has a high thermal conductivity.

[0005] JP 2000-203973 A discloses a carbon-based metal compositematerial comprising a carbonaceous matrix impregnated with at least onemetal selected from the group consisting of aluminum, magnesium, tin,zinc, copper, silver, iron, nickel and their alloys, 90% by volume ormore of voids in the carbonaceous matrix being impregnated with themetal, and the amount of the metal being 35% by volume or less of theentire carbon-based metal composite material.

[0006] JP 2001-58255 A discloses a carbon-based metal composite materialproduced by impregnating carbon moldings comprising carbon particles orfibers containing graphite crystals with aluminum, copper, silver orthese alloys at high pressure by a melt-forging method, which has athermal conductivity of 150 W/mK or more and a thermal expansioncoefficient of 4×10⁻⁶/K to 12×10⁻⁶/K in a thickness direction at roomtemperature. This carbon-based metal composite material has a structurecomprising a graphite matrix having high rigidity, a high thermalconductivity and a small thermal expansion coefficient as a skeleton,with its voids impregnated with a metal. Accordingly, it has both smallthermal expansion coefficient inherent in graphite and high thermalconductivity inherent in a metal.

[0007] Despite the above advantages, these carbon-based metal compositematerials are disadvantageous in having much larger thermal expansioncoefficients than those of silicon and compound semiconductors. Largedifferences from silicon and compound semiconductors in a thermalexpansion coefficient undesirably exert large thermal stress to CPUs orlight-emitting diodes during soldering or brazing heat-dissipatingsubstrates to the CPUs or the light-emitting diodes, or during theoperation of the CPUs or the light-emitting diodes. Accordingly, astress-relieving member is usually interposed between the CPU or thelight-emitting diode and the heat-dissipating substrate. However,because the stress-relieving member does not necessarily have asufficiently large thermal conductivity, the use of the carbon-basedmetal composite material having a high thermal conductivity for theheat-dissipating substrate does not exhibit its effect sufficiently.

[0008] Further, when the heat-dissipating substrate is bonded to the CPUor the light-emitting diode, soldering is usually conducted at about 200to 300° C. in the case of an aluminum-impregnated graphite substrate,and brazing is usually conducted at about 700 to 800° C. in the case ofa copper-impregnated graphite substrate. It has been found, however,that when exposed to such high temperature, the composite substrateimpregnated with aluminum or copper exhibits extremely different sizesdue to a residual stress before and after the heating. Such thermalhysteresis causes warp in the heat-dissipating substrate bonded to theCPU or the light-emitting diode, so that the heat-dissipating substratemay finally be broken, or that the CPU or the laser diode, etc. may alsobe damaged by thermal stress.

OBJECTS OF THE INVENTION

[0009] Accordingly, an object of the present invention is to provide acomposite material having a high thermal conductivity and as small athermal expansion coefficient as those of silicon and compoundsemiconductors, with substantially no thermal hysteresis.

[0010] Another object of the present invention is to provide aheat-dissipating substrate formed from a composite material having ahigh thermal conductivity and a small thermal expansion coefficient.

[0011] A further object of the present invention is to provide a methodfor producing such a composite material having a high thermalconductivity and a small thermal expansion coefficient.

[0012] A still further object of the present invention is to provide amethod for producing such a heat-dissipating substrate having a highthermal conductivity and a small thermal expansion coefficient.

SUMMARY OF THE INVENTION

[0013] As a result of intense research in view of the above objects, theinventor has found that by impregnating a porous graphitized extrudatewith a molten metal and then heat-treating the resultant compositematerial, it is possible to (a) increase the thermal conductivity anddecrease the thermal expansion coefficient to the same level as those ofsilicon and compound semiconductors, and (b) substantially removethermal expansion hysteresis, thereby providing the composite materialwith good dimensional stability when heated. The present invention hasbeen completed based on this finding.

[0014] Thus, the composite material of the present invention having ahigh thermal conductivity and a small thermal expansion coefficient isobtained by impregnating a porous graphitized extrudate with a metal,having such anisotropy that the thermal conductivity and the thermalexpansion coefficient are 250 W/mK or more and less than 4×10-6/K,respectively, in an extrusion direction; and that the thermalconductivity and the thermal expansion coefficient are 150 W/mK or moreand 10×10⁻⁶/K or less, respectively, in a direction perpendicular to theextrusion direction.

[0015] In a preferred embodiment of the present invention, the thermalconductivity is 250 W/mK or more in an extrusion direction and 150 W/mKor more in a direction perpendicular to said extrusion direction, andthe thermal expansion coefficient is 0.1×10⁻⁶/K or more and less than4×10⁻⁶/K in the extrusion direction and 4×10⁻⁶/K or more and 10×10⁻⁶/Kor less in the perpendicular direction.

[0016] In a preferred embodiment of the present invention, a dimensionalchange ratio due to thermal hysteresis is within ±0.1% in the extrusiondirection and in a direction perpendicular to the extrusion directionafter a heat treatment, meaning that the composite material hassubstantially no thermal expansion hysteresis.

[0017] The heat-dissipating substrate of the present invention iscomposed of the above composite material having a high thermalconductivity and a small thermal expansion coefficient, and has athickness direction substantially in alignment with the extrusiondirection of the porous graphitized extrudate, and a surfaceperpendicular to the extrusion direction, onto which a heat-generatingbody is going to be bonded.

[0018] The method for producing a composite material having a highthermal conductivity and a small thermal expansion coefficient accordingto the present invention comprises the steps of (1) graphitizing anextrudate of carbon particles and/or carbon fibers and tar pitch byburning; (2) impregnating the resultant porous graphitized extrudatewith a molten metal at high temperature and pressure; and (3)heat-treating the resultant graphite/metal composite material.

[0019] The method for producing a heat-dissipating substrate having ahigh thermal conductivity and a low thermal expansion according to thepresent invention comprises the steps of producing a composite materialhaving a high thermal conductivity and a small thermal expansioncoefficient by the above method; and then cutting the composite materialalong a surface substantially perpendicular to the extrusion directionof the porous graphitized extrudate. Said perpendicular surface ispreferably used as a surface, onto which a heat-generating body is goingto be bonded.

[0020] The thus obtained composite material having a high thermalconductivity and a small thermal expansion coefficient is cut to a plateshape having a thickness of about 0.1 to 100 mm, desirable for use asheat sinks, etc. The metal-impregnated composite material is usually cutto a plate shape after the heat treatment. However, when theheat-dissipating substrate is required to have a shape with extremelyhigh precision, the metal-impregnated composite material is preferablyheat-treated after cut to a plate shape, and cut to the target shapeagain. In any case, the heat-dissipating substrate of the presentinvention formed from the porous graphitized extrudate has a dimensionalchange ratio of within ±0.1% due to thermal hysteresis both in anextrusion direction and in a perpendicular direction. Accordingly, eventhough it is subjected to thermal stress during brazing, etc., it isfree from warp and peeling at bonding interfaces, etc. after cooling.

[0021] The composite material of the present invention having a highthermal conductivity and a small thermal expansion coefficient hasanisotropy in the thermal conductivity and the thermal expansioncoefficient. Specifically, its thermal conductivity is 250 W/mK or moreand its thermal expansion coefficient is lower than 4×10⁻⁶/K in theextrusion direction, and its thermal conductivity is 150 W/mK or moreand its thermal expansion coefficient is 10×10⁻⁶/K or less in adirection perpendicular to the extrusion direction. Accordingly, whenused for heat sinks or heat spreaders, etc. for semiconductor devices,the influence of thermal stress is suppressed, and heat spreads in alateral direction and is well conducted in a thickness direction,achieving efficient heat dissipation.

[0022] In addition, it has a small thermal expansion coefficient in thethickness direction, it exhibits high dimensional precision in thethickness direction when assembled to a package, making it possible toprovide a high-sealing package.

[0023] The preferred structure of the composite material of the presentinvention having a high thermal conductivity and a small thermalexpansion coefficient will be explained below. The composite material ofthe present invention having a high thermal conductivity and a smallthermal expansion coefficient preferably has a bulk density of 1.9 g/cm³or more, with the metal in an amount of 10 to 30% by volume.

[0024] The resistivity of the composite material having a high thermalconductivity and a small thermal expansion coefficient is preferably 4μm or less in the extrusion direction, and 7 μΩm or less in theperpendicular direction. The more preferred resistivity is 2 μΩm or lessin the extrusion direction, and 3.5 μΩm or less in the perpendiculardirection.

[0025] The metal in the composite material having a high thermalconductivity and a small thermal expansion coefficient is preferably atleast one selected from the group consisting of aluminum, copper,chromium, silver, magnesium and zinc or an alloy comprising one or moreof said metals. In a preferred embodiment, the metal is an aluminumalloy comprising 11 to 14% by mass of silicon, the balance beingsubstantially aluminum and inevitable impurities, the percentage of aneedle-shaped structure having a length of 30 μm or less and an aspectratio (length/diameter) of 10 or more being preferably 10% or less, morepreferably 5% or less, in a silicon (Si)-rich phase precipitated in themetal. The amount of oxygen in the aluminum alloy is preferably 400 ppmor less. When the metal is copper or its alloy, the amount of oxygen inthe metal is preferably 400 ppm or less, more preferably 250 ppm orless.

[0026] The porous graphitized extrudate used in the present invention ispreferably composed of carbon particles such as cokes, etc. and tarpitch, and the carbon particles preferably have an average particle sizeof 50 μm or more. The ash content of the extrudate is preferably 0.5% bymass or less, more preferably 0.3% by mass.

[0027] The resistivity of the porous graphitized extrudate used in thepresent invention is preferably less than 7 μΩm in an extrusiondirection, and 7 μΩm or more in a direction perpendicular to theextrusion direction, and the ratio of the resistivity in the extrusiondirection to that in the perpendicular direction is preferably 0.9 orless. More preferably, the resistivity of the porous graphitizedextrudate is 6 μΩm or less in the extrusion direction, and 8 μΩm or morein the direction perpendicular to the extrusion direction, and theresistivity ratio (extrusion direction/perpendicular direction) is 0.6or less.

[0028] The thermal expansion coefficient of the porous graphitizedextrudate used in the present invention is preferably 3×10⁻⁶/K or lessin the extrusion direction, and 4×10⁻⁶/K or less in the directionperpendicular to the extrusion direction, and the ratio of the thermalexpansion coefficient in the extrusion direction to that in theperpendicular direction is 0.8 or less. More preferably, the thermalexpansion coefficient of the porous graphitized extrudate is 1×10⁻⁶/K orless in the extrusion direction, and 3×10⁻⁶/K or less in the directionperpendicular to the extrusion direction, and the thermal expansioncoefficient ratio (extrusion direction/perpendicular direction) is 0.5or less.

[0029] The thermal conductivity of the porous graphitized extrudate usedin the present invention is preferably 150 W/mK or more in the extrusiondirection, and 80 W/mK or more, more preferably 100 W/mK or more, in adirection perpendicular to the extrusion direction. The ratio of thethermal conductivity in the extrusion direction to that in theperpendicular direction is preferably 1.3 or more, more preferably 1.5or more.

[0030] When the metal is aluminum or its alloy, the impregnation of theporous graphitized extrudate with the molten metal is preferablyconducted at a temperature higher than the melting point by 10° C. ormore and at a pressure of 10 MPa or more, and the heat treatment of thegraphite/metal composite material is preferably conducted under theconditions of a temperature of (the melting point −10° C.) or less and200° C. or higher, at a temperature-elevating speed of 30° C./minute orless, and a cooling speed of 20° C./minute or less. More preferably, thetemperature-elevating speed is 10° C./minute or less, and the coolingspeed is 10° C./minute or less.

[0031] When the metal is copper or its alloy, the impregnation of theporous graphitized extrudate with the molten metal is preferablyconducted at a temperature higher than the melting point by 10° C. ormore and at a pressure of 10 MPa or more, and the heat treatment of thegraphite/metal composite material is preferably conducted under theconditions of a temperature of (the melting point −10° C.) or less and300° C. or higher, at a temperature-elevating speed of 30° C./minute orless, and at a cooling speed of 20° C./minute or less. More preferably,the temperature-elevating speed is 10° C./minute or less, and thecooling speed is 10° C./minute or less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1(a) is a schematic cross-sectional view showing a dieapparatus for melt-forging a porous graphitized extrudate, whichcomprises a cavity in which a porous graphitized extrudate is placed andinto which a molten metal is poured;

[0033]FIG. 1(b) is a schematic cross-sectional view showing the dieapparatus of FIG. 1(a) for melt-forging the molten metal containing theporous graphitized extrudate;

[0034]FIG. 1(c) is a schematic cross-sectional view showing the dieapparatus of FIG. 1(a), from which a melt-forged product is taken out;

[0035]FIG. 1(d) is a cross-sectional view showing a metal-impregnatedporous graphitized extrudate cut out from the melt-forged product;

[0036]FIG. 2 is a graph schematically showing a heat treatment patternconducted on the graphite/metal composite material of the presentinvention;

[0037]FIG. 3 is a schematic cross-sectional view showing theheat-dissipating substrate of the present invention, to which asemiconductor device is bonded;

[0038]FIG. 4(a) is a SIM photograph showing the graphite/Al—Si compositematerial before the heat treatment in Example 1;

[0039]FIG. 4(b) is a SIM photograph showing the graphite/Al—Si compositematerial after the heat treatment in Example 1;

[0040]FIG. 5(a) is a graph showing the thermal expansion hysteresis ofthe graphite/Al—Si composite material before the heat treatment in itsextrusion direction in Example 1;

[0041]FIG. 5(b) is a graph showing the thermal expansion hysteresis ofthe graphite/Al—Si composite material after the heat treatment in itsextrusion direction in Example 1;

[0042]FIG. 6(a) is a graph showing the thermal expansion hysteresis ofthe graphite/Al—Si composite material before the heat treatment in theperpendicular direction in Example 1;

[0043]FIG. 6(b) is a graph showing the thermal expansion hysteresis ofthe graphite/Al—Si composite material after the heat treatment in theperpendicular direction in Example 1;

[0044]FIG. 7(a) is a SIM photograph showing the graphite/coppercomposite material before the heat treatment in Example 2;

[0045]FIG. 7(b) is a SIM photograph showing the graphite/coppercomposite material after the heat treatment in Example 2;

[0046]FIG. 8(a) is a graph showing the thermal expansion hysteresis ofthe graphite/copper composite material before the heat treatment in itsextrusion direction in Example 2;

[0047]FIG. 8(b) is a graph showing the thermal expansion hysteresis ofthe graphite/copper composite material after the heat treatment in itsextrusion direction in Example 2;

[0048]FIG. 9(a) is a graph showing the thermal expansion hysteresis ofthe graphite/copper composite material before the heat treatment in theperpendicular direction in Example 2;

[0049]FIG. 9(b) is a graph showing the thermal expansion hysteresis ofthe graphite/copper composite material after the heat treatment in theperpendicular direction in Example 2;

[0050]FIG. 10 is cross-sectional view showing an example of asemiconductor module comprising a heat-dissipating substrate formed bythe graphite/metal composite material of the present invention;

[0051]FIG. 11 is a perspective view showing an example of a reinforcingpipe member used as a substrate with throughholes;

[0052]FIG. 12(a) is a cross-sectional view taken along the line A-A inFIG. 11;

[0053]FIG. 12(b) is a cross-sectional view showing a state where a metalpipe member is being fitted in a throughhole of the heat-dissipatingsubstrate shown in FIG. 11;

[0054]FIG. 12(c) is a cross-sectional view showing another example of ametal pipe member fitted in the throughhole of the heat-dissipatingsubstrate;

[0055]FIG. 12(d) is a perspective view showing a further example of ametal pipe member fitted in the throughhole of the heat-dissipatingsubstrate; and

[0056]FIG. 12(e) is a perspective view showing a still further exampleof a metal pipe member fitted in the throughhole of the heat-dissipatingsubstrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] [1] Composite Material Having High Thermal Conductivity and SmallThermal Expansion Coefficient

[0058] (A) Structure

[0059] (1) Porous Graphitized Extrudate

[0060] The porous graphitized extrudate used in the present inventionpreferably has a bulk density of 2 g/cm³ or less, particularly 1.6 to1.95 g/cm³. When the bulk density is more than 2 g/cm³, the impregnationof a molten metal is insufficient, failing to obtain sufficient effectsof improving its thermal conductivity. On the other hand, when the bulkdensity is less than 1.6 g/cm³, the graphite skeleton has insufficientstrength, so that the thermal expansion coefficient of the entirecomposite material is largely influenced by the thermal expansioncoefficient of a metal. The more preferred bulk density of the porousgraphitized extrudate is 1.65 to 1.85 g/cm³.

[0061] (2) Impregnating Metal

[0062] A molten metal impregnated into the porous graphitized extrudateis preferably at least one metal selected from the group consisting ofaluminum, copper, chromium, silver, magnesium and zinc or an alloycomprising one or more of the metals. Particularly, the aluminum alloyis preferably an aluminum-silicon alloy containing 11 to 14% by mass ofsilicon. The reason therefor seems to be that the inclusion of 11 to 14%by volume of silicon lowers the melting point of a molten metal, therebysuppressing the generation of aluminum carbide, and thus preventingdecrease in thermal conductivity. An additional reason seems to be thatneedle-shaped silicon particles contained in the alloy are spheroidizedby the heat treatment, resulting in a decreased heat resistance and thusimproved thermal conductivity.

[0063] The copper alloy is preferably a chromium-copper alloy. Thereason therefor seems to be that chromium contained in the alloyimproves the strength of an interface between graphite and copper,resulting in improvement in the strength of the composite material. Thecontent of chromium is 0.1 to 10% by mass, preferably 0.1 to 5% by mass,more preferably 0.1 to 2% by mass.

[0064] The percentage of the metal in the composite material ispreferably 10 to 30% by volume. When the amount of the metalimpregnating the porous graphitized extrudate is less than 10% by volumeof the composite material, the bulk density of the composite material isless than 1.9 g/cm³, resulting in insufficient effects of improving thethermal conductivity by metal impregnation. On the other hand, when themetal is more than 30% by volume, the amount of the impregnating metalis too much to the graphite skeleton, so that the thermal expansioncoefficient of the entire composite material is largely influenced bythe thermal expansion coefficient of the metal, resulting in too largedifference in a thermal expansion coefficient from silicon and compoundsemiconductors. The more preferred percentage of the metal in thecomposite material is 5 to 25% by volume.

[0065] Because the voids of the porous graphitized extrudate are filledwith the metal as densely as possible, the bulk density of the compositematerial is preferably 1.9 g/cm³ or more. When the bulk density of thecomposite material is less than 1.9 g/cm³, the composite material hastoo much a void ratio, failing to sufficiently improve the thermalconductivity. On the other hand, when the bulk density is more than 5g/cm³, the high-pressure impregnation process needs too strictconditions of temperature and pressure, making the production of thecomposite material difficult and thus costly. The more preferred bulkdensity of the composite material is 1.9 to 4 g/cm³.

[0066] (B) Production Method

[0067] (1) Production of Porous Graphitized Extrudate

[0068] The porous graphitized extrudate per se can be produced by knownmethods. Typically, carbon materials such as cokes, etc. are pulverizedand classified to a proper size, and then melt-blended with pitch as abinder. The resultant blend is extruded from a die orifice having apredetermined shape, cut to a predetermined length and then burned forgraphitization. In place of the carbon powder, carbon fibers may beused, and a mixture of carbon powder and carbon fibers may be used.

[0069] The average particle size of the carbon powder such as cokepowder, etc. is preferably 50 μm or more. When the average particle sizeof the carbon powder is less than 50 μm, the resultant porousgraphitized extrudate has an insufficient thermal conductivity. When theaverage particle size of the carbon powder is more than 3 mm, theresultant porous graphitized extrudate disadvantageously hasinsufficient mechanical strength. The more preferred average particlesize of the carbon powder is about 50 μm to about 3 mm. The carbonfibers are preferably pitch carbon fibers with an average length ofabout 50 μm to about 5 mm.

[0070] A mixing ratio of carbon powder and/or carbon fibers to pitch ispreferably 10:1 to 10:4, more preferably 10:2 to 10:3, by weight. Whenthe mixing ratio is less than 10:1 or more than 10:4, the resultantblend has improper viscosity, resulting in difficulty in extrusion.

[0071] The melt blend of carbon powder and/or carbon fibers and tarpitch is extruded from the die at a temperature of 100° C. to 140° C.

[0072] To obtain the graphite/metal composite material having excellentthermal conductivity and thermal expansion coefficient, the porousgraphitized extrudate is preferably composed of high-purity graphite.

[0073] Specifically, an ash content in the porous graphitized extrudateis preferably 0.5% by mass or less, more preferably 0.3% by mass orless.

[0074] The porous graphitized extrudate is burned at 700° C. to 1000° C.after extrusion. Because the burned molded body has many voids, pitch isintroduced into the voids of the burned molded body and burned again, toachieve a bulk density of 1.65 g/cm³ or more. Thereafter, the moldedbody is heat-treated at a temperature of 2600° C. to 3000° C. forgraphitization, which turns carbon to graphite, thereby providing aporous graphitized extrudate. To obtain the graphite/metal compositematerial having a high thermal conductivity and a small thermalexpansion coefficient, incombustible mineral materials (ash content)remaining after burning the porous graphitized extrudate should be 0.5%by mass or less, namely, the porous graphitized extrudate should beturned to high-purity graphite.

[0075] (2) Impregnation of Metal

[0076] The impregnation of the porous graphitized extrudate with amolten metal can be conducted by a melt-forging method. One example ofdie apparatuses suitable for conducting the melt-forging method is shownin FIG. 1. As shown in FIG. 1(a), the die apparatus 1 comprises an upperdie portion 11 having a center cavity 11 a, a lower die portion 12disposed under the upper die portion 11 and having a center opening 12a, a lower punch 13 disposed in a cavity 11 a of the upper die portion11, a shaft 14 connected to the bottom of the lower punch 13 and passingthrough an opening 12 a of the lower die portion 12, an upper punch 15entering into the cavity 11 a of the upper die portion 11, and a plungershaft 16 connected to an upper surface of the upper punch 15.

[0077] As shown in FIG. 1(a), the upper punch 15 is removed, and theporous graphitized extrudate 20 is placed on the lower punch 13, whichis lowered to the bottom of the cavity 11 a of the upper die portion 11.In this state, a molten metal M is poured from a ladle 2 into the cavity11 a. At this time, it is preferable that the upper and lower dieportions 11, 12, the porous graphitized extrudate, etc. are heated to apredetermined temperature in advance, and that the molten metal M in asufficient amount is poured into the cavity 11 a to prevent thesolidification of the molten metal M during the impregnation. To preventthe porous graphitized extrudate 20 from floating while pouring themolten metal, a weight made of iron, etc. is more preferably placed onthe porous graphitized extrudate 20.

[0078] As shown in FIG. 1(b), the upper punch 15 is caused to enter intothe cavity 11 a to press the molten metal M via the plunger shaft 16 ata high pressure, such that the high-pressure molten metal M penetratesinto the voids of the porous graphitized extrudate 20. After the moltenmetal M entering into the porous graphitized extrudate 20 is solidified,as shown in FIG. 1(c), the upper punch 15 is removed, and the lowerpunch 13 is then elevated, to take out the resultant metal-impregnatedporous graphitized extrudate 21. Finally, the metal-impregnated porousgraphitized extrudate 21 is cut out from the solidified metal M′ asshown in FIG. 1(d). To prevent the molten metal M from being solidifiedbefore fully entering into the voids of the porous graphitized extrudate20 under high pressure, it is preferable to heat the upper and lower dieportions 11, 12 and the upper and lower punches 13, 15 at apredetermined temperature during the melt-forging.

[0079] The melt-forging temperature is preferably higher than themelting point of the molten metal by 10° C. or more, though it may varydepending on the types of the molten metal. Specifically, themelt-forging temperature of each metal or its alloy is as shown in Table1 below. When the melt-forging temperature is lower than the lowertemperature limit in any molten metal, the intrusion of the molten metalinto the voids of the porous graphitized extrudate is insufficient.Effects obtained by elevating the melt-forging temperature aresubstantially saturated at the upper temperature limit, and furtherimprovement in the effects cannot be obtained even by elevating themelt-forging temperature. The temperature of the porous graphitizedextrudate is preferably heated at a temperature equal to the meltingpoint of the molten metal, particularly at a temperature higher than themelting point of the molten metal in advance, in any molten metal beforethe impregnation, because such heating makes it possible to fullyimpregnate the voids of the extrudate with the molten metal. TABLE 1Melt-Forging Temperature (° C.) Molten Metal⁽¹⁾ Preferable Range MorePreferable Range Aluminum  600 to 900  700 to 900 Copper 1100 to 14001200 to 1400 Silver 1000 to 1300 1100 to 1300 Magnesium  700 to 900  750to 900 Zinc  500 to 800  600 to 800

[0080] The melt-forging pressure should be 10 MPa or more regardless ofthe types of the molten metal. It is more preferably 50 MPa or more. Ifthe melt-forging pressure were lower than the lower temperature limit,the intrusion of the molten metal into the voids of the porousgraphitized extrudate would be insufficient in any molten metal. Effectsobtained by elevating the melt-forging pressure are substantiallysaturated at the upper pressure limit, and further improvement in theeffects cannot be obtained by elevating the melt-forging pressure.

[0081] The pressing time may generally be 1 to 30 minutes regardless ofthe type, temperature and pressure of the molten metal. When thepressing time is less than 1 minute, the porous graphitized extrudate isnot fully impregnated with the molten metal. On the other hand, when itis more than 30 minutes, the temperature of the molten metal becomes toolow, failing to achieve further impregnation.

[0082] (3) Heat Treatment

[0083]FIG. 2 shows a preferable heat treatment pattern for thegraphite/metal composite material of the present invention. Thetemperature-elevating speed of the graphite/metal composite material ispreferably 30° C./minute or less, more preferably 10° C./minute or less.When the temperature-elevating speed is more than 30° C./minute, thetemperature of the composite material does not become uniform. The lowerlimit of the temperature-elevating speed may be about 0.5° C./minute,taking into account the efficiency of the heat treatment.

[0084] The holding temperature of the graphite/metal composite materialis preferably (the melting point of each metal −10° C.) or less and 200°C. or higher. When the holding temperature is higher than the meltingpoint of each metal −10° C., the metal is softened or melted, so that itis likely to elute from the porous graphitized extrudate. On the otherhand, when the holding temperature is lower than 200° C., sufficientheat treatment effects cannot be obtained. The holding time may be about1 to 120 minutes.

[0085] Because the graphite/metal composite material held at the abovetemperature is preferably cooled slowly, its cooling speed is preferably20° C./minute or less, more preferably 10° C./minute or less. When thecooling speed is more than 20° C./minute, thermal hysteresis remains inthe impregnating metal. The lower limit of the cooling speed may beabout 0.5° C./minute, taking into account the efficiency of the heattreatment.

[0086] The heat treatment may be conducted on the metal-impregnatedporous graphitized extrudate 21, and the metal-impregnatedgraphite/metal composite material cut along a surface perpendicular tothe extrusion direction may be heat-treated. The former is morepreferable from the aspect of the production process.

[0087] The preferred heat treatment conditions of each metal-impregnatedgraphite/metal composite material are shown in Table 2 below. TABLE 2Heat Treatment Conditions⁽¹⁾ of Graphite/Metal Composite MaterialTemperature- Impregnating Elevating Holding Cooling Speed Metal Speed (°C.) Temperature (° C.) (° C.) Aluminum 0.5 to 30  200 to 550 0.5 to 20 (2 to 10) (450 to 550)  (2 to 10) Copper 0.5 to 30  300 to 1000 0.5 to20  (2 to 10) (800 to 1000)  (2 to 10) Silver 0.5 to 30  300 to 900 0.5to 20  (2 to 10) (700 to 900)  (2 to 10) Magnesium 0.5 to 30  200 to 6500.5 to 20  (2 to 10) (550 to 650)  (2 to 10) Zinc 0.5 to 30  200 to 4500.5 to 20  (2 to 10) (300 to 450)  (2 to 10)

[0088] (C) Properties

[0089] (1) Thermal Conductivity

[0090] Because the graphite/metal composite material of the presentinvention has a structure, in which a high-thermal-conductivity metalhas entered into the voids of the porous graphitized extrudate at a highpressure, it has an extremely high thermal conductivity due to thegraphite. In addition, because the graphite skeleton per se hasanisotropy because of its structure as an extrudate, it has differentthermal conductivities in the extrusion direction and a directionperpendicular thereto. The porous graphitized extrudate per se has athermal conductivity of 150 W/mK or more in the extrusion direction, and80 W/mK or more in the perpendicular direction. Accordingly, thecomposite material has a thermal conductivity of 250 W/mK or more in theextrusion direction, and 150 W/mK or more in the perpendiculardirection, regardless of the type of the impregnating metal. Further,the feature of the present invention is that the thermal conductivity ofthe graphite/metal composite material is further improved by a heattreatment.

[0091] The thermal conductivity of each graphite/metal compositematerial before and after the heat treatment is shown in Table 3 below.TABLE 3 Thermal Conductivity (W/mK)⁽¹⁾ of Graphite/Metal CompositeMaterial Before and After Heat Treatment Impregnating In Extrusion InPerpendicular Metal Direction Direction Aluminum After: 280-340 After:230-300 Before: 240-300 Before: 220-260 Copper After: 290-350 After:240-300 Before: 240-300 Before: 230-270 Silver After: 290-350 After:240-300 Before: 240-300 Before: 230-270 Magnesium After: 250-300 After:190-250 Before: 210-240 Before: 140-180 Zinc After: 250-300 After:180-250 Before: 200-230 Before: 130-170

[0092] (2) Thermal Expansion Coefficient

[0093] Because the graphite/metal composite material of the presentinvention has a skeleton constituted by a porous graphitized extrudate,it has as a whole a thermal expansion coefficient close to that ofgraphite. Also, because the graphite skeleton is constituted by theextrudate, there are differences in a thermal expansion coefficientbetween the extrusion direction and the direction perpendicular thereto.The thermal expansion coefficient of the porous graphitized extrudateper se is 3.0×10⁻⁶/K or less in the extrusion direction and 4.0×10⁻⁶/Kor less in the perpendicular direction. Accordingly, though slightlydifferent depending on the type of the impregnating metal, the thermalexpansion coefficient of the composite material is as small as less than4.0×10⁻⁶/K in the extrusion direction, and 10×10⁻⁶/K or less in theperpendicular direction expansion coefficient. Further, the feature ofthe present invention is that the thermal expansion coefficient of thegraphite/metal composite material is further lowered by a heattreatment.

[0094] The thermal expansion coefficient and dimensional change ratio ofeach graphite/metal composite material before and after the heattreatment are shown in Table 4 below. TABLE 4 Thermal ExpansionCoefficient (×10⁻⁶/K)⁽¹⁾ of Graphite/Metal Impreg- Composite MaterialBefore and After Heat Treatment nating In Extrusion In PerpendicularDimensional Metal Direction Direction Change Ratio (%)⁽²⁾ Alu- After:0.7-3.9 After: 4.0-10.0 After: 0.1 or less minum Before: 4.1-10.0Before: 10.5-20.0 Before: >0.1 Copper After: 0.7-3.5 After: 4.0-10.0After: 0.1 or less Before: 5.0-9.0 Before: 10.5-15.0 Before: >0.1 SilverAfter: 0.7-3.9 After: 4.1-10.0 After: 0.1 or less Before: 4.1-12.0Before: 10.5-21.0 Before: >0.1 Mag- After: 1.0-3.9 After: 5.0-10.0After: 0.1 or less nesium Before: 4.1-15.0 Before: 10.5-25.0Before: >0.1 Zinc After: 1.0-3.9 After: 5.0-10.0 After: 0.1 or lessBefore: 4.1-18.0 Before: 10.5-28.0 Before: >0.1

[0095] (3) Thermal Expansion Hysteresis

[0096] The thermal expansion of the graphite/metal composite materialbefore the heat treatment has a thermal hysteresis. Namely, thegraphite/metal composite material before the heat treatment has suchpoor dimensional stability that when it is heated and then cooled toroom temperature, it does not return from a thermally expanded state tothe original size. It has been found, however, that the graphite/metalcomposite material is provided with an extremely reduced dimensionalchange ratio by the heat treatment according to the present invention.The graphite/metal composite material with an excellent dimensionalstability suffers substantially no dimensional change when it is usedfor a heat-dissipating substrate and thus subjected to heat by solderingor brazing. Accordingly, the heat-dissipating substrate does not warp,so that unnecessary stress is not applied to heat-generating devicessuch as semiconductor devices or laser devices.

[0097] (4) Other Properties

[0098] The resistivity of the graphite/metal composite material slightlydecreases by the heat treatment. Decrease in the resistivity isremarkable particularly in the extrusion direction. The resistivity ofthe graphite/metal composite material is preferably 4 μΩm or less in theextrusion direction and 7 μΩm or less in the perpendicular direction.The resistivity of each graphite/metal composite material before andafter the heat treatment is shown in Table 5 below. TABLE 5 Resistivity(μΩm) of Graphite/Metal Composite Material Before and After HeatTreatment⁽¹⁾ Impregnating In Extrusion In Perpendicular Metal DirectionDirection Aluminum After: 1.0-4.0 After: 1.0-7.0 Before: 1.0-4.1 Before:1.0-7.1 Copper After: 1.0-3.0 After: 1.0-5.0 Before: 1.0-3.1 Before:1.0-5.1 Silver After: 1.0-3.0 After: 1.0-5.0 Before: 1.0-3.0 Before:1.0-5.1 Magnesium After: 1.0-4.0 After: 1.0-7.0 Before: 1.0-4.1 Before:1.0-7.1 Zinc After: 1.0-3.0 After: 1.0-7.0 Before: 1.0-4.1 Before:1.0-7.1

[0099] It is presumed that the decrease of the resistivity of thegraphite/metal composite material by the heat treatment is caused by thefact that the heat treatment lowers the amount of oxygen in theimpregnating metal, increasing the purity of the metal. The amount ofoxygen in the impregnating metal is different depending on the type ofthe metal. In the graphite/metal composite material before the heattreatment, the amount of oxygen is generally 200 to 400 ppm in aluminumor its alloy, 500 to 1000 ppm in copper or its alloy, 200 to 600 ppm insilver or its alloy, 200 to 600 ppm in magnesium or its alloy, and 500to 2000 ppm in zinc or its alloy. Particularly in the case of copper orits alloy, the amount of oxygen extremely decreases by the heattreatment. Specifically, the amount of oxygen is as small as 400 ppm orless in the graphite/copper composite material after the heat treatment.

[0100] The Young's modulus of the graphite/metal composite material doesnot substantially change before and after the heat treatment, and it isgenerally 5 GPa or more in a surface direction, on a necessary level foruse in the heat-dissipating substrates. Also, the bending strength ofthe graphite/metal composite material does not substantially changebefore and after the heat treatment, and it is generally 10 MPa or more,on a necessary level for use in the heat-dissipating substrates. Theincrease of a thermal conductivity, the reduction of a thermal expansioncoefficient and the improvement of dimensional stability by the heattreatment seem to be mainly due to the fact that a residual strain atthe time of melt forging disappears by the heat treatment. Particularlyin the case of using an Al—Si alloy, the spheroidization of theneedle-shaped structure having high heat resistance by the heattreatment seems to contribute to the improvement of the thermalconductivity. Also, in the case of using copper or its alloy, thereduction of the amount of oxygen functioning to increase heatresistance by the heat treatment seems to contribute to the improvementof the thermal conductivity.

[0101] [2] Heat-Dissipating Substrate

[0102] The heat-dissipating substrate is obtained by cutting theheat-treated graphite/metal composite material to a predetermined size.Though the heat-dissipating substrate is preferably used as a heat sinkor a heat spreader, etc., it may have a structure in which aheat-dissipating fan and a heat spreader are integrated because ofexcellent workability of the graphite/metal composite material. Asurface of the heat-dissipating substrate, onto which a heat-generatingdevice such as a semiconductor device or a laser device, etc. is bonded,is preferably perpendicular to the extrusion direction of thegraphite/metal composite material, though it may be in parallel with theextrusion direction.

[0103] As shown in FIG. 3, for instance, when the surface for bonding asemiconductor device 3 is perpendicular to the extrusion direction ofthe heat-dissipating substrate 4, heat is quickly conducted from thesemiconductor device 3 to a heat sink 5 bonded to the other surface ofthe heat-dissipating substrate 4, because the heat-dissipating substrate4 has a thermal conductivity larger in the thickness direction than inthe surface direction. Because the heat-dissipating substrate 4 has athermal expansion coefficient larger in the surface direction than inthe thickness direction, the thermal expansion coefficient of theheat-dissipating substrate 4 in the surface direction is close to thoseof both of the semiconductor device 3 and the heat sink 5. Accordingly,no large thermal stress is applied to both interfaces between theheat-dissipating substrate 4 and the semiconductor device 3 and betweenthe heat-dissipating substrate 4 and the heat sink 5 during theoperation of the semiconductor device 3.

[0104] The semiconductor device 3 is not subjected to a large thermalstress by heating during soldering or brazing for bonding thesemiconductor device 3 to the heat-dissipating substrate 4. Further,because the thermal expansion coefficient of the heat-dissipatingsubstrate in the thickness direction is as small as half or less of thethermal expansion coefficient in the surface direction, theheat-dissipating substrate preferably exhibits a small expansion ratioin the height direction during heating for producing a package, so thatit is easily positioned in an assembling process.

[0105] The method for producing the heat-dissipating substrate ischaracterized in that the graphite/metal composite material is cut afterthe heat treatment. If the graphite/metal composite material were cutbefore the heat treatment, because of poor dimensional stability, theheat-dissipating substrate would be subjected to dimensional change byheating during soldering or brazing or by temperature elevation duringthe operation. Accordingly, finish working may be needed to achievedimensional precision, or the bonding interfaces of the heat-dissipatingsubstrate may have poor reliability.

[0106] The heat-dissipating substrate cut to a predetermined size ispreferably coated with a metal layer to secure the sealing of a package.Though the metal layer is usually formed on the entire surface of theheat-dissipating substrate, the sealing of the package can be secured byforming the metal layer at least on a surface for mounting asemiconductor device, etc. (and a rear surface). The sealing is enoughwhen the amount of a helium gas leaked is 1×10⁻² Pa·cm³/s or less.

[0107] The method of forming the metal layer may be a CVD method, avapor deposition method, a sputtering method, a metal pasteprinting/baking method, a plating method, etc. To achieve enoughsealing, the thickness of the metal layer is preferably 0.5 μm to 10 μm.In the case of plating, electroless plating is more preferable thanelectroplating, because the electroless plating can form a uniform metallayer on the heat-dissipating substrate.

[0108] The plating layer is preferably Ni—P, Ni—B, Cu, etc. When theimpregnating metal is copper or its alloy, requiring a heat resistanceof 700° C. or higher, the Ni—B plating is particularly preferablebecause it is less diffusion-reactive to the metal and thus stable. Themetal layer may be utilized not only for sealing, but also as a primerfor adhesion to other parts. Such metal layer desirably improves theadhesion to heat-generating bodies such as semiconductor devices andpackages.

[0109] Because the heat-dissipating substrate comprising aluminum or itsalloy as an impregnating metal has a large thermal conductivity, athermal expansion coefficient close to those of silicon and compoundsemiconductors, and good solderability, it is suitable for heatspreaders, etc. for semiconductor devices using soldering for bonding.In addition, because the graphite/aluminum composite material has lowmelt-forging temperature and heat-treating temperature, it isadvantageous in low production cost. Because the heat-dissipatingsubstrate of the present invention has a thermal expansion coefficientcloser to those of semiconductor devices and is lighter in weight thanconventional heat spreaders composed of copper or aluminum, it isdesirable for heat spreaders with grease.

[0110] The heat-dissipating substrate comprising copper or its alloy asan impregnating metal has a large thermal conductivity, a small thermalexpansion coefficient, and good dimensional stability. In addition,because it is impregnated with copper having a relatively high meltingpoint, it has high heat resistance, undergoing no change at a brazingtemperature. Accordingly, it is suitable for applications such asoptical transmission packages, etc. including the heat dissipation oflaser devices brazed with silver brazing alloys.

[0111] In the case of a heat-dissipating substrate having throughholesfor fastening, a metal pipe member fitted in each throughhole acts as areinforcing member, preventing damage such as cracking even when a highfastening torque is applied. Thus, a high fastening torque can beobtained by this fastening structure. The metal pipe member also acts asa heat-conductive member for dispersing a thermal stress concentratedaround each throughhole, thereby improving the function of theheat-dissipating substrate.

[0112] Because the heat-dissipating substrate comprising silver or itsalloy as an impregnating metal is impregnated with silver having a largethermal conductivity and a melting point between those of aluminum andcopper, it is suitable for applications requiring a heat resistance ofabout 900° C.

[0113] Because the heat-dissipating substrate comprising magnesium orits alloy as an impregnating metal is impregnated with magnesium havinga melting point between those of aluminum and silver, it is suitable forparts requiring a higher heat resistance than that of aluminum.

[0114] Because the heat-dissipating substrate comprising zinc or itsalloy as an impregnating metal has a larger thermal expansioncoefficient than those of substrates impregnated with aluminum, copper,silver, magnesium, etc., it is suitable for bonding to parts havinglarge thermal expansion coefficients, for instance, bonding to heatsinks of aluminum or copper. In addition, it is advantageous in lowproduction cost, because of its low impregnation temperature.

[0115] The present invention will be explained in detail referring toExamples below without intention of restricting the present inventionthereto.

EXAMPLE 1

[0116] Using a porous graphitized extrudate having a specific bulkdensity of 1.70, an ash content of 0.3% by mass, resistivity of 5.0 μΩmand 8.5 μΩm, respectively, in an extrusion direction and in thedirection perpendicular to the extrusion direction, a thermal expansioncoefficient of 0.6×10⁻⁶/K and 2.0×10⁻⁶/K, respectively, in an extrusiondirection and in the direction perpendicular to the extrusion direction,a thermal conductivity of 230 W/mK and 120 W/mK, respectively, in anextrusion direction and in the direction perpendicular to the extrusiondirection, which was obtained by extruding and graphitizing a melt blendof coke particles having an average particle size of 500 μm and pitch,and an Al—Si alloy containing 12% by mass of Si, a graphite/Al—Sicomposite material was produced under the following conditions.

[0117] After the above Al—Si alloy melt (750° C.) was poured into acavity of a die apparatus (held at 750° C.) shown in FIG. 1(a), in whichthe above porous graphitized extrudate was placed, an upper punch waslowered to conduct melt-forging at 100 MPa for 5 minutes. An excessportion of the Al—Si alloy was cut off to obtain the graphite/Al—Sicomposite material. This graphite/Al—Si composite material was subjectedto a heat treatment under the following conditions.Temperature-elevating speed: 2° C./minute Holding conditions: 500° C. ×60 minutes Cooling speed: 2° C./minute

[0118] The graphite/Al—Si composite material after the heat treatmentwas cut to a size of 40.0 mm×20.0 mm×2.0 mm as a sample for aheat-dissipating substrate. The thickness direction of theheat-dissipating substrate was in alignment with the extrusion directionof the composite material.

[0119] Samples obtained from the graphite/Al—Si composite materialsbefore and after the heat treatment were measured with respect to theamount of the Al—Si alloy, the amount of a needle-shaped structure inthe Si-rich phase in the Al—Si alloy, a bulk density, a thermalconductivity, a thermal expansion coefficient, resistivity, a Young'smodulus, a bending strength, and a dimensional change ratio by thefollowing methods. The measurement results are shown in Table 6 below.

[0120] (1) The bulk density was an apparent weight per a unit volume.

[0121] (2) The thermal conductivity was measured by a thermal constantanalyzer TC-7000H by a laser flash method available from ULVAC-RIKO,Inc. according to JIS R 1611.

[0122] (3) The thermal expansion coefficient and the dimensional changeratio were measured by a thermomechanical analyzer using a thermalanalysis system EXSTAR6000 available from Seiko Instruments Inc.

[0123] (4) The resistivity was measured by a four-terminal method usingZEM-2 available from ULVAC-RIKO, Inc.

[0124] (5) The Young's modulus was measured by a two-probe methodreceiving an ultrasonic transmission wave using UVM-2 and a digitaloscilloscope.

[0125] (6) The bending strength was measured by a three-point bendingtest method using an autograph AG-G available from Shimadzu Corporationaccording to JIS R 1601.

COMPARATIVE EXAMPLE 1

[0126] A graphite/Al—Si composite material was produced and evaluated inthe same manner as in Example 1 except for conducting a heat treatmentunder the following conditions. The results are shown in Table 6 below.Temperature-elevating speed: 2° C./minute, Holding conditions: 150° C. ×60 minute, and Cooling speed: 2° C./minute.

[0127] TABLE 6 Properties of Graphite/Al—Si Composite MaterialComparative Measure- Example 1 Example 1 ment Before After Before AfterItems Measured Direction HT⁽¹⁾ HT HT HT Ai—Si Alloy (vol. %)⁽²⁾ — 14 1414 14 Amount of Needle- — 90 90 90 5 Shaped Structure (%) Bulk Density(g/cm³) — 2.2 2.2 2.2 2.1 Thermal Conductivity ED⁽³⁾ 300 300 300 340(W/mK) PD⁽⁴⁾ 220 220 220 250 Thermal Expansion ED 8.1 8.0 8.1 3.5Coefficient (×10⁻⁶/K) PD 17.8 17.0 17.8 6.9 Dimensional Change ED 0.180.18 0.18 0.01 Ratio (%) PD 0.32 0.32 0.32 0.01 Resistivity (μΩm) ED 1.41.4 1.4 1.2 PD 1.7 1.7 1.7 1.8 Young's Modulus ED 18 18 18 18 (GPa) PD11 11 11 11 Bending Strength ED 47 46 47 42 (MPa) PD 29 29 29 26 Amountof Oxygen in — 250 250 250 200 Al—Si alloy (ppm)

[0128] As is clear from Table 6, the heat treatment increased thethermal conductivity of the graphite/Al—Si composite material, whileextremely decreasing the thermal expansion coefficient and dimensionalchange ratio thereof. There was substantially no change before and afterthe heat treatment in the resistivity, the Young's modulus and thebending strength. The above results indicate that the heat treatmentprovided the graphite/Al—Si composite material with desirable propertiesfor heat-dissipating substrates. In Comparative Example 1, on the otherhand, there was substantially no change before and after the heattreatment in the thermal conductivity, the thermal expansion coefficientand the dimensional change ratio.

[0129] With respect to samples of the graphite/Al—Si composite materialsbefore and after the heat treatment, the structures of their Al-Siregions were observed by a scanning ion microscope (SIM) FB-2000Aavailable from Hitachi, Ltd. The SIM photographs are shown in FIGS. 4(a)and 4(b). As is clear from FIG. 4(a), a needle-shaped structure composedof a Si-rich phase was precipitated in a sample of the graphite/Al—Sicomposite material before the heat treatment. In a sample composed ofthe graphite/Al—Si composite material after the heat treatment, on theother hand, the needle-shaped structure became spheroidal as is clearfrom FIG. 4(b). In this Example, the percentage (surface area ratio inthe photomicrograph) of the needle-shaped structure having a length of30 μm or less and an aspect ratio (length/diameter) of 10 or more amongthe silicon-rich phase decreased to 5%. It is presumed thatspheroidization decreases the heat resistance of thelow-thermal-conductivity Si-rich phase, contributing to increase in thethermal conductivity of the Si-rich phase. It has been found that whenthe surface area ratio of the needle-shaped structure having a length of30 μm or less and an aspect ratio of 10 or more among the silicon-richphase becomes 10% or less, particularly 5% or less, the thermalconductivity of the graphite/Al—Si composite material extremelyincreases.

[0130] Samples composed of the graphite/Al—Si composite materials beforeand after the heat treatment were heated to temperatures ranging fromroom temperature to 500° C., and left to cool, their hysteresis ofthermal expansion was measured in the extrusion direction and aperpendicular direction. The results are shown in FIGS. 5 and 6. As isclear from FIGS. 5(a) and 6(a), the graphite/Al—Si composite materialbefore the heat treatment exhibited a dimensional change ratio of 0.18%in the extrusion direction and 0.32% in the perpendicular directionafter the thermal hysteresis. In the graphite/Al—Si composite materialafter the heat treatment, on the other hand, as is clear from FIGS. 5(b)and 6(b), there was as small a dimensional change ratio as 0.01% in anyof the extrusion direction and the perpendicular direction after thethermal hysteresis, indicating that there was substantially nodimensional change. The above results indicate that the graphite/Al—Sicomposite material of the present invention heat-treated after the metalimpregnation suffered from little dimensional change even after thethermal hysteresis, excellent in dimensional stability.

EXAMPLE 2

[0131] Using the same porous graphitized extrudate as in Example 1 andpure copper (purity 99.9% or more), a graphite/copper composite materialwas produced as follows. After a melt (1350° C.) of the above purecopper poured into a cavity of the die apparatus (held at 1000° C.)shown in FIG. 1(a), in which the above porous graphitized extrudate wasplaced, an upper punch was lowered to conduct melt-forging at 100 MPafor 5 minutes. Excess pure copper was cut away to obtain thegraphite/copper composite material. This graphite/copper compositematerial was subjected to a heat treatment under the followingconditions. Temperature-elevating speed: 5° C./minute, Holdingconditions: 900° C. × 120 minutes, and Cooling speed: 5° C./minute.

[0132] The graphite/copper composite material after the heat treatmentwas cut to a size of 40.0 mm×20.0 mm×2.0 mm to provide aheat-dissipating substrate sample. The thickness direction of theheat-dissipating substrate was in alignment with the extrusion directionof the composite material. Each sample was measured with respect to theamount of copper, a bulk density, a thermal conductivity, a thermalexpansion coefficient, a dimensional change ratio (after heating andheat dissipation described below), resistivity, Young's modulus, abending strength and the amount of oxygen in copper in the same manneras in Example 1. The measurement results are shown in Table 7 below.

COMPARATIVE EXAMPLE 2

[0133] A graphite/copper composite material was produced and measured inthe same manner as in Example 2 except for using a porous graphitizedextrudate having a specific bulk density of 1.58, an ash content of 0.7%by mass, resistivity of 9.0 μΩm and 9.5 μΩm, respectively, in anextrusion direction and in the direction perpendicular to the extrusiondirection, a thermal expansion coefficient of 4.0×10⁻⁶/K and 4.2×10⁻⁶/K,respectively, in an extrusion direction and in the directionperpendicular to the extrusion direction, and a thermal conductivity of130 W/mK and 80 W/mK, respectively, in an extrusion direction and in thedirection perpendicular to the extrusion direction. The measurementresults are shown in Table 7 below. TABLE 7 Properties ofGraphite/Copper Composite Material Comparative Measure- Example 2Example 2 ment Before After Before After Items Measured Direction⁽²⁾HT⁽¹⁾ HT HT HT Amount of Copper — 35 35 16 16 (vol. %)⁽¹⁾ Bulk Density(g/cm³) — 4.5 4.5 3.2 3.1 Thermal Conductivity ED⁽³⁾ 160 180 300 350(W/mK) PD⁽⁴⁾ 130 150 220 250 Thermal Expansion ED 12.0 7.0 6.1 1.7Coefficient (×10⁻⁶/K) PD 13.5 8.0 11.9 5.8 Dimensional Change ED 0.600.05 0.35 0.02 Ratio (%) PD 0.50 0.03 0.40 0.01 Resistivity (μΩm) ED10.0 10.0 1.6 1.1 PD 11.0 11.0 1.7 1.8 Young's Modulus ED 25 25 15 16(GPa) PD 20 20 10 10 Bending Strength ED 45 44 38 30 (MPa) PD 35 32 2420 Amount of Oxygen in — 1000 400 850 150 Copper (ppm)

[0134] As is clear from Table 7, the heat treatment provided thegraphite/copper composite material with a slightly increased thermalconductivity and an extremely decreased thermal expansion coefficient.With respect to the resistivity, the Young's modulus and the bendingstrength, there was substantially no change before and after the heattreatment. The above results indicate that the graphite/copper compositematerial got desirable properties for heat-dissipating substrates by theheat treatment. On the other hand, the graphite/copper compositematerial of Comparative Example 2 had a low thermal conductivity and alarge thermal expansion coefficient even after the heat treatment,though its dimensional change ratio decreased by the heat treatment.

[0135] With respect to samples composed of the graphite/copper compositematerials before and after the heat treatment, a copper region wasobserved by a scanning ion microscope (SIM). The SIM photograph is shownin FIG. 7. As is clear from FIG. 7(a), a sample composed of thegraphite/copper composite material before the heat treatment had acopper phase with a typical structure. On the other hand, as is clearfrom FIG. 7(b), a sample composed of the graphite/copper compositematerial after the heat treatment had a copper phase changed to anequiaxial crystal, with oxygen in an extremely reduced amount. It ispresumed that decrease in the amount of oxygen reducing heat resistanceresults in the improvement of a thermal conductivity.

[0136] Samples composed of the graphite/copper composite materialsbefore and after the heat treatment were heated to temperatures rangingfrom room temperature to 900° C. and then left to cool to measure athermal expansion hysteresis in both extrusion and perpendiculardirections. The results are shown in FIGS. 8 and 9. As is clear fromFIGS. 8(a) and 9(a), the graphite/copper composite material before theheat treatment had a dimensional change ratio of 0.35% and 0.40%,respectively, in an extrusion direction and a perpendicular directionafter cooling. And as is clear from FIGS. 8(b) and 9(b), thegraphite/copper composite material after the heat treatment had adimensional change ratio of 0.02% and 0.01%, respectively, in anextrusion direction and a perpendicular direction, with substantially nodimensional change. This reveals that the graphite/copper compositematerial of the present invention has a small dimensional change andthus excellent dimensional stability even after heating.

EXAMPLE 3

[0137] Using the same porous graphitized extrudate as in Example 1having a specific bulk density of 1.70, an ash content of 0.3% by mass,resistivity of 5.0 μΩm and 8.5 μΩm, respectively, in an extrusiondirection and in the direction perpendicular to the extrusion direction,a thermal expansion coefficient of 0.6×10⁻⁶/K and 2.0×10⁻⁶/K,respectively, in an extrusion direction and in the directionperpendicular to the extrusion direction, and a thermal conductivity of230 W/mK and 120 W/mK, respectively, in an extrusion direction and inthe direction perpendicular to the extrusion direction, and brasscomprising 70% by mass of Cu and 30% by mass of Zn, a graphite/brasscomposite material was produced under the following conditions.

[0138] After a melt (1350° C.) of the above brass was poured into acavity of the die apparatus (held at 1000° C.) shown in FIG. 1(a), inwhich the above graphite was placed, an upper punch was lowered toconduct melt-forging at 100 MPa for 5 minutes. Excess pure copper wascut away to obtain the graphite/brass composite material. Thisgraphite/brass composite material was subjected to a heat treatmentunder the following conditions. Temperature-elevating speed: 5° C./minute, Holding conditions: 900° C. × 120 minutes, and Cooling speed: 5°C./minute.

[0139] The resultant graphite/brass composite material sample wasmeasured with respect to the amount of brass, a bulk density, a thermalconductivity, a thermal expansion coefficient, a dimensional changeratio (after heating and heat dissipation), resistivity, a Young'smodulus, a bending strength, and the amount of oxygen in brass in thesame manner as in Example 1. The measurement results are shown in Table8 below.

COMPARATIVE EXAMPLE 3

[0140] A graphite/brass composite material was produced and measured inthe same manner as in Example 3 except for conducting a heat treatmentunder the following conditions. The results are shown in Table 8.Temperature-elevating speed: 5° C./minute, Holding conditions: 250° C. ×120 minutes, and Cooling speed: 5° C./minute.

[0141] TABLE 8 Properties of Graphite/Brass Composite Material Measure-Comparative ment Before Example 3 Example 3 Items Measured directionHT⁽¹⁾ (After HT) (After HT) Amount of Brass 22 22 22 (vol. %)⁽²⁾ BulkDensity (g/cm³) — 3.2 3.2 3.1 Thermal Conductivity ED⁽³⁾ 280 280 330(W/mK) PD⁽⁴⁾ 180 180 220 Thermal Expansion ED 7.1 7.1 2.0 Coefficient(×10⁻⁶/K) PD 13.1 13.0 6.5 Dimensional Change ED 0.32 0.30 0.02 Ratio(%) PD 0.38 0.36 0.01 Resistivity (μΩm) ED 1.8 1.9 1.7 PD 1.9 1.9 1.8Young's Modulus (GPa) ED 15 15 16 PD 10 10 10 Bending Strength (MPa) ED38 38 36 PD 24 24 23 Amount of Oxygen in — 850 850 150 Brass (ppm)

EXAMPLE 4

[0142] A heat-dissipating substrate of 40.0 mm×20.0 mm×2.0 mm cut outfrom the graphite/Al—Si alloy composite material (after the heattreatment) of Example 1 shown in Table 6 was subjected to electrolessNi—P plating after a zincate treatment. A heat-dissipating substrate of40.0 mm×20.0 mm×2.0 mm cut out from the graphite/Cu composite material(after the heat treatment) of Example 2 shown in Table 7 was alsosubjected to electroless Ni—B plating. To evaluate the correlation ofthe presence of a plating layer and sealability, the amount of a heliumgas passing through each plated heat-dissipating substrate was measuredby a helium leak detector DLMS-33 available from ULVAC, Inc. accordingto JIS C 7021 A-6. The amount of a helium gas passing through the platedheat-dissipating substrate, as the amount of a leaked helium gas, wasused as a parameter of sealability. The results are shown in Table 9below. TABLE 9 Amount of Leaked Type of Thickness Helium Gas Type ofSubstrate Plating (μm) (×10⁻²Pa · cm³/s) Graphite/Al-12Si Non — 1.0Graphite/Al-12Si Ni—P 2 <0.1 Graphite/Cu Non — 30 Graphite/Cu Ni—B 40.50 Graphite/Cu Ni—B 6 <0.10 Graphite/Cu Cu 5 0.35

[0143] It has been found that any of the graphite/Al-12Si and thegraphite/Cu was extremely improved in sealability by plating.Incidentally, when the plating thickness is less than 0.5 μm, sufficientsealability cannot be obtained. On the other hand, when the platingthickness is more than 20 μm, the remaining stress increases, resultingin peeling of the plating layer. The graphite/Al-12Si compositematerial, etc. are preferable in high adhesion to a vapor-deposited Allayer. The graphite/Cu composite material, etc. may be printed with anAg paste and then sintered at 900° C. In this case, because of a lowfilm stress, the film thickness may be as large as about 30 μm.

EXAMPLE 5

[0144]FIG. 10 shows an example of a module for mounting semiconductordevices, which comprises a heat-dissipating substrate 4 a of 100 mm×100mm×2 mm formed by the graphite/metal (Cu) composite material of thepresent invention, an insulating substrate 6 formed by a silicon nitrideplate of 30 mm×30 mm×0.8 mm, and a heat sink 5. The insulating substrate6 was brazed to the heat-dissipating substrate 4 a, and after platingthe insulating substrate 6 with Ni, semiconductor chips 3 of 10 mm×10 mmwere soldered to the insulating substrate 6. The heat-dissipatingsubstrate 4 a and the heat sink 5 were mechanically fastened by bolts,etc. via a high-thermal-conductivity grease to provide a module.

[0145] The graphite/Cu composite material used in this Example to formthe heat-dissipating substrate 4 a was impregnated with ancopper-chromium alloy containing 1.0% by mass of Cr. Theheat-dissipating substrate 4 a had a thermal conductivity of 250 W/mK ormore in a thickness direction and 150 W/mK or more in a directionperpendicular to the thickness direction (on the side of thedevice-mounting surface), and a thermal expansion coefficient of morethan 0.1×10⁻⁶/K and less than 4×10⁻⁶/K in the thickness direction and4×10⁻⁶/K or more and 10×10⁻⁶/K or less in the perpendicular direction.

[0146] The heat-dissipating characteristics of the semiconductor modulein this Example were evaluated by measuring the surface temperature ofeach semiconductor chip 3 and thermal resistance (° C./W) between thesemiconductor chip 3 and the rear surface of the heat sink 5 duringsupplying current, and further by measuring thermal resistance betweenthe semiconductor chip 3 and the rear surface of the heat sink 5 after3000 cycles of a heating/cooling test from −40° C. to 125° C. Thethermal resistance after 3000 cycles of the temperature cycle test isexpressed by an increment (%) relative to the thermal resistance beforethe temperature cycle test.

[0147] It was thus found that the surface temperature of thesemiconductor devices was 52.1° C., that the thermal resistance was0.23° C./W, and that the increment after the cycle test was 2.5%,indicating that the surface temperature of the semiconductor devices andthe thermal resistance were lower, and the increment after the cycletest was smaller than those of conventional ones.

[0148] When the graphite/metal composite material of the presentinvention is used for a heat-dissipating substrate 4 a, theheat-dissipating substrate 4 a may have throughholes for fastening aheat sink 5 as in the above embodiment. In this case, a reinforcingmetal pipe member 7 is preferably fitted in each throughhole 40. Thoughnot particularly restrictive, the pipe member 7 may have such a shape asshown in FIGS. 12(a) and (b), or may have a flange 70 as shown in FIG.12(c), as long as it is fitted in the throughhole 40. Flanges 70provided on both ends preferably further prevent cracks from generatingfrom the throughholes 40. When the metal pipe member 7 has a flange 70only on one end, the flange 70 is positioned such that it is in contactwith a bolt head.

[0149] As shown in FIG. 12(d), the metal pipe member 7 may have a slit71. Further, as shown in FIG. 12(e), the metal pipe member 7 may have anotch 72. The slit 71 and the notch 72 enable the metal pipe member 7 toelastically deform in a circumferential direction, thereby making iteasy to fit the metal pipe member 7 in the throughhole 40. The slit 71and the notch 72 have a function to buffer a load applied to thesubstrate due to the expansion of the pipe member 7, which is caused bythe heat generation of the substrate. Accordingly, they provide themetal pipe member 7 with sufficient durability to a cooling/heatingcycle, a soldering reflow process, etc. The metal bonding such asbrazing between the pipe member 7 and the substrate preferably improvestheir adhesion, resulting in improved heat dissipation.

[0150] As described above in detail, the graphite/metal compositematerial of the present invention has (a) both properties of graphite(small thermal expansion coefficient) and properties of a metal (largethermal conductivity), because it comprises a graphite skeleton and ahigh-thermal-conductivity metal entering into the voids of the graphiteskeleton, and (b) a thermal conductivity slightly improved and a thermalexpansion coefficient extremely decreased by the heat treatment. Inaddition, because the graphite/metal composite material of the presentinvention has a skeleton of a graphitized extrudate, there aredifferences in properties between an extrusion direction and aperpendicular direction. Accordingly, with a cutting direction inparallel with the extrusion direction or the perpendicular directiondepending on applications, it is possible to obtain the heat-dissipatingsubstrate having desired thermal conductivity and thermal expansioncoefficient. Further, because the thermal expansion hysteresis isreduced to substantially zero by the heat treatment, the graphite/metalcomposite material of the present invention advantageously has excellentdimensional accuracy even after soldering or brazing.

1. A composite material having a high thermal conductivity and a smallthermal expansion coefficient, which is obtained by impregnating aporous graphitized extrudate with a metal; said composite materialhaving such anisotropy that said thermal conductivity and said thermalexpansion coefficient are 250 W/mK or more and less than 4×10⁻⁶/K,respectively, in an extrusion direction; and that said thermalconductivity and said thermal expansion coefficient are 150 W/mK or moreand 10×10⁻⁶/K or less, respectively, in a direction perpendicular tosaid extrusion direction.
 2. A composite material having a high thermalconductivity and a small thermal expansion coefficient, which isobtained by impregnating a porous graphitized extrudate with a metal; adimensional change ratio due to thermal hysteresis being within ±0.1% inthe extrusion direction and in a direction perpendicular to saidextrusion direction after a heat treatment.
 3. The composite materialhaving a high thermal conductivity and a small thermal expansioncoefficient according to claim 1, wherein it has a bulk density of 1.9g/cm³ or more; and wherein the amount of said metal is 10 to 30% byvolume.
 4. The composite material having a high thermal conductivity anda small thermal expansion coefficient according to claim 1, wherein itsresistivity is 4 μΩm or less in said extrusion direction and 7 μΩm orless in said perpendicular direction.
 5. The composite material having ahigh thermal conductivity and a small thermal expansion coefficientaccording to claim 1, wherein said metal is at least one metal selectedfrom the group consisting of aluminum, copper, chromium, silver,magnesium and zinc, or an alloy comprising one or more of said metals.6. The composite material having a high thermal conductivity and a smallthermal expansion coefficient according to claim 5, wherein said metalis an aluminum alloy comprising 11 to 14% by mass of silicon, thebalance being substantially aluminum and inevitable impurities, andwherein the percentage of a needle-shaped structure having a length of30 μm or less and an aspect ratio (length/diameter) of 10 or more is 10%or less among a silicon (Si)-rich phase precipitated in its metalstructure.
 7. The composite material having a high thermal conductivityand a small thermal expansion coefficient according to claim 5, whereinsaid metal is copper or its alloy; and wherein the amount of oxygen insaid metal is 400 ppm or less.
 8. A method for producing a compositematerial having a high thermal conductivity and a small thermalexpansion coefficient, comprising the steps of (1) graphitizing anextrudate of carbon particles and/or carbon fibers and tar pitch byburning; (2) impregnating the resultant porous graphitized extrudatewith a molten metal at high temperature and pressure; and (3)heat-treating the resultant graphite/metal composite material.
 9. Themethod for producing a composite material having a high thermalconductivity and a small thermal expansion coefficient according toclaim 8, wherein said extrudate comprises carbon particles and tarpitch; and wherein said carbon particles have an average particle sizeof 50 μm or more and an ash content of 0.5% by mass or less.
 10. Themethod for producing a composite material having a high thermalconductivity and a small thermal expansion coefficient according toclaim 8, wherein said porous graphitized extrudate has resistivity ofless than 7 μΩm in the extrusion direction and 7 μΩm or more in adirection perpendicular to said extrusion direction; and wherein theratio of said resistivity in said extrusion direction to that in saidperpendicular direction is 0.9 or less.
 11. The method for producing acomposite material having a high thermal conductivity and a smallthermal expansion coefficient according to claim 8, wherein the thermalexpansion coefficient of said porous graphitized extrudate is 3×10⁻⁶/Kor less in the extrusion direction and 4×10⁻⁶/K or less in a directionperpendicular to said extrusion direction; and wherein the ratio of saidthermal expansion coefficient in said extrusion direction to that insaid perpendicular direction is 0.8 or less.
 12. The method forproducing a composite material having a high thermal conductivity and asmall thermal expansion coefficient according to claim 8, wherein thethermal conductivity of said porous graphitized extrudate is 150 W/mK ormore in the extrusion direction and 80 W/mK or more in a directionperpendicular to said extrusion direction; and wherein the ratio of saidthermal conductivity in said extrusion direction to that in saidperpendicular direction is 1.3 or more.
 13. The method for producing acomposite material having a high thermal conductivity and a smallthermal expansion coefficient according to claim 8, wherein said metalis at least one metal selected from the group consisting of aluminum,copper, chromium, silver, magnesium and zinc or an alloy comprising oneor more of said metals.
 14. The method for producing a compositematerial having a high thermal conductivity and a small thermalexpansion coefficient according to claim 13, wherein said metal isaluminum or its alloy; and wherein the impregnation of said porousgraphitized extrudate with said molten metal is conducted at atemperature higher than the melting point of said molten metal by 10° C.or more and at a pressure of 10 MPa or more.
 15. The method forproducing a composite material having a high thermal conductivity and asmall thermal expansion coefficient according to claim 13, wherein saidmetal is copper or its alloy; and wherein the impregnation of saidporous graphitized extrudate with said molten metal is conducted at atemperature higher than the melting point of said molten metal by 10° C.or more and at a pressure of 10 MPa or more.
 16. The method forproducing a composite material having a high thermal conductivity and asmall thermal expansion coefficient according to claim 13, wherein saidmetal is aluminum or its alloy; and wherein the heat treatment of saidgraphite/metal composite material is conducted under the conditions of atemperature of (the melting point −10° C.) or less and 200° C. orhigher, a temperature-elevating speed of 30° C./minute or less, and acooling speed of 20° C./minute or less.
 17. The method for producing acomposite material having a high thermal conductivity and a smallthermal expansion coefficient according to claim 13, wherein said metalis copper or its alloy; and the heat treatment of said graphite/metalcomposite material is conducted under the conditions of a temperature of(the melting point −10° C.) or less and 300° C. or higher, atemperature-elevating speed of 30° C./minute or less, and a coolingspeed of 20° C./minute or less.
 18. A heat-dissipating substratecomprising the composite material having a high thermal conductivity anda small thermal expansion coefficient recited in claim 1, wherein thethickness direction of said substrate is substantially in alignment withthe extrusion direction of said porous graphitized extrudate; andwherein it has a surface perpendicular to said extrusion direction, ontowhich a heat-generating body is going to be bonded.
 19. Theheat-dissipating substrate according to claim 18, wherein a metal layerhaving such sealability that the amount of a helium gas leaked is 1×10⁻²Pa·cm³/s or less is formed on at least on a surface, onto which aheat-generating body is going to be bonded.
 20. The heat-dissipatingsubstrate according to claim 19, wherein said metal layer comprises aNi—B plating layer and/or a Ni—P plating layer each having a thicknessof 0.5 to 20 μm.
 21. The heat-dissipating substrate according to claim18, wherein said substrate has throughholes, in each of which areinforcing pipe member is fitted.
 22. A method for producing aheat-dissipating substrate having a high thermal conductivity and a lowthermal expansion, comprising the steps of producing a compositematerial having a high thermal conductivity and a small thermalexpansion coefficient by the method recited in claims 8; and thencutting said composite material along a surface substantiallyperpendicular to the extrusion direction of said porous graphitizedextrudate.
 23. The method for producing a heat-dissipating substrateaccording to claim 22, wherein said perpendicular surface is used as asurface, onto which a heat-generating body is going to be bonded. 24.The method for producing a heat-dissipating substrate according to claim22, comprising the steps of cutting said heat-dissipating substrate, andthen forming a metal layer at least on a surface, onto which aheat-generating body is going to be bonded.
 25. The composite materialhaving a high thermal conductivity and a small thermal expansioncoefficient according to claim 2, wherein it has a bulk density of 1.9g/cm³ or more; and wherein the amount of said metal is 10 to 30% byvolume.
 26. The composite material having a high thermal conductivityand a small thermal expansion coefficient according to claim 2, whereinits resistivity is 4 μΩm or less in said extrusion direction and 7 μΩmor less in said perpendicular direction.
 27. The composite materialhaving a high thermal conductivity and a small thermal expansioncoefficient according to claim 2, wherein said metal is at least onemetal selected from the group consisting of aluminum, copper, chromium,silver, magnesium and zinc, or an alloy comprising one or more of saidmetals.
 28. The composite material having a high thermal conductivityand a small thermal expansion coefficient according to claim 27, whereinsaid metal is an aluminum alloy comprising 11 to 14% by mass of silicon,the balance being substantially aluminum and inevitable impurities, andwherein the percentage of a needle-shaped structure having a length of30 μm or less and an aspect ratio (length/diameter) of 10 or more is 10%or less among a silicon (Si)-rich phase precipitated in its metalstructure.
 29. The composite material having a high thermal conductivityand a small thermal expansion coefficient according to claim 27, whereinsaid metal is copper or its alloy; and wherein the amount of oxygen insaid metal is 400 ppm or less.
 30. A heat-dissipating substratecomprising the composite material having a high thermal conductivity anda small thermal expansion coefficient recited in claim 2, wherein thethickness direction of said substrate is substantially in alignment withthe extrusion direction of said porous graphitized extrudate; andwherein it has a surface perpendicular to said extrusion direction, ontowhich a heat-generating body is going to be bonded.
 31. Theheat-dissipating substrate according to claim 30, wherein a metal layerhaving such sealability that the amount of a helium gas leaked is 1×10⁻²Pa·cm³/s or less is formed on at least on a surface, onto which aheat-generating body is going to be bonded.
 32. The heat-dissipatingsubstrate according to claim 30, wherein said metal layer comprises aNi—B plating layer and/or a Ni—P plating layer each having a thicknessof 0.5 to 20 μm.
 33. The heat-dissipating substrate according to claim30, wherein said substrate has throughholes, in each of which areinforcing pipe member is fitted.